It is known that system designs for high power flash lamps typically include the following components: 1/Lamp envelope or lamp tube made of tubular material with adequate transparency for the desired spectral emission band(s) (e.g., UV-grade quartz for UV radiation), and filled with gas or gases such as xenon, krypton, or other suitable gas(es); 2/Electrodes located in opposite ends of the tube, connected to a source of high voltage and producing an electrical discharge in the gas(es); 3/Surrounding jacket or second tube of suitably transparent material around the circumference of the lamp envelope, providing a volume for circulation of cooling fluid (gas or liquid) between the lamp exterior surfaces and the internal surface of the jacket. Such cooling fluid providing removal of excess heat developed during the lamp operation.
While there are many known styles and methods for operating pulsed flash lamps, it is most common for high power pulsed lamp operation to encompass some version of the three typical operating modes: an ignition mode, a simmer mode, and a pulse mode. The ignition mode provides initial ionization of gas inside the tube by a special igniter. The simmer (standby) mode is provided by a small current that supports a constant low level of gas ionization inside the tube. The pulse mode is produced by a short, high peak power and high voltage discharge inside the tube, the discharge having a duration between microseconds and milliseconds, and developing pulses with peak power from one to hundreds of megawatts.
The growing demand by new applications for increased UV processing power has in many instances required much improved flash lamp performance over the capabilities of the generation of PUV lamps prior to this invention. Compared to previous pulsed lamp designs, this new generation of high power and performance pulsed lamps is physically characterized by a much longer length anode-to-cathode spacing (for example, by a factor of three or more), with a subsequent increase in the length, weight, and aspect ratio profile of the lamp. Compared to previous pulsed lamp designs, this new generation of high power and performance pulsed lamps is electrically characterized by pulses with larger currents (peak and/or average magnitude), longer arc lengths (anode-to-cathode spacing), and higher required operating voltages. In order to extend both power and performance capabilities beyond the pre-existing generation of so-called medium-to-high power flash lamps, new methods and designs are required. For example, large-scale water disinfection and remediation is just one application where the older generation of PUV lamps have shown to be lacking, and therefore not considered by industry to be entirely suitable to the task. A new generation of higher power and performance pulsed ultraviolet lamps is both desirable and advantageous. UV light can effectively disinfect across a broad range of targeted pathogens. In sharp contrast with chemical disinfectants such as chlorine, UV light can disinfect without adversely affecting the taste, odor, or safety of the water, and is particularly effective against protozoa, such as Cryptosporidium Parvum. Additionally, pulsed UV systems in particular advantageously can deliver a consistent UV light output efficiency despite any lamp and/or ambient temperature changes, and instant UV power “ON” and “OFF” cycling, instantly variable and precise levels of UV power output throughout the range of zero to 100%. Importantly, PUV can do so with neither the hazardous mercury, nor the explosive potential created by high lamp envelope temperatures and pressures that characterize conventional continuous wave (CW) medium pressure UV lamps. Furthermore, it is known that the CW mercury lamps (among others) have an inherent problem of performance degradation due to thermal gradient induced fouling (minerals attraction) of lamp cooling jackets. Therefore, it is advantageous to create pulsed UV systems with the capability to fulfill the requirements of large-scale UV processing applications.
It is known by practitioners of the art that the previous generation of PUV lamps, while demonstrating very attractive potential advantages and benefits, have never successfully been deployed on a large scale, and were seemingly relegated to laboratory work and/or relatively low power niche applications. Known problems have included unacceptably short service life, uncompetitive electrical-to-optical output efficiency, inconsistent UV output, and UV spectral and power outputs that are not well-matched to the targeted application. The records show that lamp service lives were limited by one or more combinations of rapidly-declining UV output, excessive lamp aging that degraded and then prematurely prevented operation, and/or catastrophic failure of the lamp envelope material. Electrical-to-UV output efficiencies were within the range of 5% to 9%, which compares unfavorably with the approximate practical range of 17% to 35% typical of CW mercury UV lamps. The UV output of the previous generation of PUV lamps became progressively less consistent (in terms of energy per pulse and spectral characteristics) with eventually unsuccessful attempts to push towards higher output powers.
The primary reason for these limiting problems is that neither the lamp designs, nor the pulsed power supply designs, are substantively different from the conventional flash lamp technology that has been in use for many decades in relatively lower performance systems. A thorough survey of prior art reveals that there exist no novel departures from standard pulsed lamp designs that enable scaling of the technology into the performance and power levels that today are desirable for certain applications. Indeed, the designs of pulsed lamp systems that fail to meet the more recently extended performance criteria are, in essence, identical to the designs traditionally used in smaller, less demanding, and lower performance systems.
Practitioners of the art are aware of the long-established body of knowledge concerning the various standard techniques for designing and driving pulsed flash lamps. While these techniques tend to work well within the broad base of established applications for which these designs have been incorporated, it is now known that the simple extension of these standard designs and methods into the more demanding class of very high power PUV lamps has been shown to be insufficient for the task.
In order to achieve the potential advantages of very high power pulsed UV lamps, it is necessary to create new and unique lamp designs by which this technology is enabled, thereby inventing a whole new generation of higher capability and performance pulsed lamps. The design methods for the older generation of lower performance and power flash lamps are inadequate to the task; this invention provides necessary solutions.
There are multiple causes for the potentially deleterious stress to which this new generation of high performance pulsed lamps may be subjected, such as compression and tension induced stress, thermal expansion and contraction induced stress, tensile stress resulting from induced deformations, asymmetrical heating and deformation of the envelope resulting in a bending of the lamp envelope, and resonance oscillations.
For example, a typical characteristic of pulsed flash lamps is that, beginning with the onset of the main current pulse, the discharge consists of a thin cathode sheath (cathode “glow”, negative glow, and so-called “dark spaces”) and a positive column that fills most of the anode-to-cathode space. At the higher lamp pressures, this cathode sheath is less than a micron thick, but has a pressure, applied voltage, and current-independent voltage drop of approximately 150 Volts. Although the sheath-dissipated power is small because of the shallow depth of the sheath, the power dissipated per unit volume is very high, resulting in instantaneously high temperatures and pressures, and the subsequent formation of a strong shock wave. This initial strong shock wave is attenuated within a few millimeters, depositing much of its energy in the region surrounding the electrode, including the lamp envelope. The power of the main pulse that is subsequently deposited into the main column between the anode and cathode rapidly heats the plasma along the length of the bore, thereby creating a cylindrical shock wave that travels to the envelope wall, reflecting and oscillating several times at very high acoustic frequencies (≈100 kHz).
According to both theoretical calculations of and empirical data from pulsed flash lamp operation, very high power pulses can produce high forces that create compression and tension stresses in lamp materials. In particular, the high power pulses produce gas heating and pressure increase, axial and radial forces, and shock waves through the gas and tube walls. As a result: 1/axial waves propagate through the gas and envelope, completely or partially reflected from tube ends and can produce a set of multiple reflected waves that interfere and create standing waves and stress points in the envelope walls; and 2/radial waves propagate through the gas, envelope walls, cooling fluid and cooling jacket, traversing through boundaries with different material properties, completely or partially reflected back and create standing waves and various stress points in the envelope walls.
Thermal expansion and contraction induced stress is created due to fast pulse gas heating that produces transient thermal loading upon the inner layer of the lamp envelope. The envelope outer layer is cooled down by outside coolant flow, which results in a temperature gradient through the tube walls and additional pulse tension stress in the envelope outer layer.
Deformations in the envelope material can result from high peak inner pressures, combined with heating and softening of the envelope inner layer. Fast cooling of the thermally-conductive quartz or glass produces hardening of deformed material and creation of compression stress in inner layers along with tension stress in outer layers of the envelope. This effect is similar to the known method of treatment of artillery cannon barrels (autofrettage) when high internal hydraulic pressure improves the barrel resistance during firing. Very small changes during each short pulse can accumulate and produce sufficient tensile stress in the tube outer layer, tube elongation and bending, which could become an additional source of tensile stress on the bulging side.
Emanating from the plasma and external lamp wiring, and in some designs also affected by surrounding component layout, high current-induced electromagnetic fields can produce asymmetrical shifting of the plasma filament away from the lamp axis and toward one side of the envelope wall. This can result in asymmetrical heating and deformation of the envelope. Accumulation of deformations and stress after multiple pulses can result in eventual bending of the lamp envelope.
Lastly, multiple high power pulse sequencing with constantly changing pulse repetition frequencies from single to thousands per second (depending on system design and operating conditions) can create a resonance effect in lamps with natural frequencies in the same range. The move towards the use of dramatically longer length lamps aggravates this situation. Resonance oscillations in a lamp can produce detrimental pulsing tension and compression stresses in lamp components. These and other mechanisms of stress development can accumulate in lamp envelope material(s) and work in combination. It is known that tube-shaped materials (quartz or glass) behave much like other hard and brittle substances; they work very well under compression, but are very sensitive to tension stress. Multiple tension cycles exceeding a critical level of stress can be responsible for a gradual development and emergence of micro-cracks in the material, leading to catastrophic breakage of the lamp. Another effect of stress and micro-cracks accumulation is the degradation of tube transparency (increased absorption of radiation by the envelope walls), and subsequent reduction of lamp electrical-to-optic output efficiency.
There is therefore a need for a reliable and cost-effective lamp system design and method of manufacture that can prevent lamp breakage and/or premature degradation of desired radiation output.